I'm connecting a 13" MacBook Air to a Dell 22" screen using the Apple Mini DisplayPort to DVI Adapter, with an Apple Keyboard with Numeric Keypad plugged into a USB on the MacBook and a USB mouse plugged into the keyboard. Also, the MacBook is running off external power.
When I close the MacBook lid, I thought the Dell monitor would become primary and the MacBook screen real-estate would "go away." However, the MacBook screen remains primary and I can move the mouse pointer off the Dell screen and onto the invisible MacBook screen.
Am I misunderstanding how clamshell mode works?